25 November 2010

Unfortunately I broke my fibula in March (gymnastics) and did a lot of damage to the various talo-fibula ligaments in the process. Those ligaments are still healing, and one part of my rehab is to do a lot of pseudo-barefoot work, mostly by wearing my Chaco sandles and Vibram KSOs during day-to-day activities. At the very least, I am trying to restore proper biomechanics to my feet, if I have to go through all this discomfort!

However as winter approached I was feeling a little trepidation, since Vibrams are totally unsuited to -20 °C weather conditions with wind and snow. The obvious pick, for me, was to look for some minimalist running flats and stuff some warm socks into them. I'm generally not willing to buy footwear without putting it on first (with the exception of thermo-molded footwear like ski boots), so I headed down to my local eclectic running store, FastTrax, and went through their stock of running flats. Eventually the salesman let me try on the garish blue shoes on the top shelf, the Inov-8 F-lite 230s, which I immediately liked. They are so named for their weight, the shoes as a pair weighs in at 230 grams, or about half a pound. From what I can tell, I'm pretty damn lucky to be able to try these on in a store in Canada as they seem to be hard to find outside of the UK.

'Garish' describes the vivid blue colour quite well

The F-lite 230s is marketed as a 'mountain running flat.' I consider them an ultra-light approach shoe. The lugs on the sole are made of sticky climbing shoe rubber, and they are quite pliable. The heel-lift is probably 3 mm, which makes them pretty much the flattest true shoe I've ever owned. I found them to be immediately comfortable out of the box. The heel box is especially effective in trapping my heel and because the sole is so flexible, there's no noticeable sliding of the heel when walking or running. These shoes are very flexible: it's easy to twist them through 180° or touch the toe to the heel.

The upper is composed of mesh, but the laces are then vertically reinforced by a pliable plastic that helps distribute the forces better so the upper doesn't collapse onto your foot. I think this is a good design decision for light-weight uppers as compared to just a velcro strap.

The most 'non-barefoot style' features of these shoes is: 1.) their general squishiness, and 2.) the pointed, low-volume toe-box.

I find the pointed toe-box a little strange. I probably sized these shoes half-a-size too large as a result; according to the Inov sizing chart I should be wearing size UK6.5 but I actually have size UK7.5s. Realistically, I should probably size for UK7.0. The pointed toe-box does seem to expand out without really putting pressure on my toes, so that's something to consider. I also remember my Vibrams were tight in the right big toe (my right foot is longer than the left) as well and they stretched out.

The shoes also have quite a thick (3 mm) and squishy insole. They can be removed of course, but for now I'm using them. In general if I take the insoles out and just walk around the shoes do not feel nearly as squishy anymore. If I did try to size down into a UK6.5 shoe, I would probably take the insoles out in the store. The finishing of the midsole isn't free of stiching so these shoes probably can't be worn sans insole and sans socks.

The last drawback of these shoes is that the lugs, being composed of climbing rubber, aren't super durable. Inov does have very similar shoes with harder wearing rubber compounds, but I didn't have any choice of models in the store.

It's sort of difficult for me to properly review these things given how I can't really run properly yet, but I have been running in them three times now. Twice was just running to school (about 2 km), which is on pavement. I do not heel strike in these shoes on pavement, even with my limited range of motion in my right ankle. The third time was a trail run at the bottom of the scramble shown in the above picture, and they performed spectacularly well in the soft trail. Foot sensitivity is not as apparent as in the Vibrams, in that small pebbles are unnoticeable but the larger rocks and branches are still felt through the flexible sole just fine, and the lugs make them grip far better in soft ground.

Up on 'Vision Quest' in the David Thompson range.

I have to say, Inov has a very extensive stable of minimalist running shoes designed for natural activities. I find their product line personally pretty compelling (in that I sort of want to buy one of each). If you like hiking and other non-road running activities, go take a look at what they offer on their website. I hope that my local shop brings in some more models, and hopefully I can try a smaller size come summer-time.

16 November 2010

A month ago a report appeared in Science magazine on a new prototype solar cell that could produce more than one electron, a packet of electrical energy, from one photon, a packet of light energy. This trick isn't new, I discussed a bunch of such concepts years ago in a post on quantum photovoltaics. What's new is that Samber et al. report in "Multiple Exciton Collection in a Sensitized Photovoltaic System," have achieved high efficiency in this process.

The typical goal for these sort of 'quantum' solar projects is to better match the wavelengths of light the photovoltaic system can absorb to that of the spectrum of light that is produced by the sun (and filtered by the atmosphere). The theoretical limit for Silicon alone in a photovoltaic system is 33.7 %, which is known as the Shockley-Quiesser limit. Of course, most commercial PV systems are only around 12 % efficient or less, so there is still considerable room for growth. An example of such a spectrum-matched system is Spectrolab's space photovoltaics, which are actually stacks of multiple photovoltaic systems all operating at different wavelengths, and achieve roughly 30 % efficiency in a commercial product.

Figure 1: The air-mass 1.5 standard spectra (indigo line), which is an estimate for the spectra that arrives at the Earth's surface after passing through the atmosphere. The red line is the band-gap for Silicon; for longer wavelengths (red-er) Si cannot absorb the photon. For shorter (blue-er) wavelengths the extra energy is lost.

If you could get one electron out from a infrared photon and two from a blue photon, then the energy gained is fairly significant. This result has been reported on before, but in this paper, for the first time, the overall system produced more electrons out than photons in, as observed to just isolated nanoparticles. In photovoltaic parlance, the number of electrons produced per photon is known as the quantum yield, but this doesn't describe how many electrons actually escape and result in electrical current. That quantity has the wordy name, absorbed photon-to-current efficiency.

Figure 2: Absorbed photon-to-current efficiency (APCE) as a function of wavelength for the described cells. The x-axis is in electron volts, to convert to wavelength, divide 1240 nm by the photon energy [Fig 4. from Sambur et al., 2010].

Overall,this would be a really interesting paper except for one obvious drawback: the active layer of quantum dots is really really thin. There's only a single layer of quantum dots and given an average diameter of 10 nm, that's not very think for visibile light. Compare that to a standard Silicon cell being hundreds of microns, or a thin-film cell at 10 μm which is still a thousand times thicker. In fact it's so thin that they absorb only 1-2 % of the incoming light.

Since the problem with quantum dot strategies is always getting the electrons out, and not absorbing the light, I'm not sure that this work will have a great impact when scaled up. At the very least, however, it does show that it's possible to build a quantum dot solar cell that works as advertised, producing more than one electron per photon, and doing that quite well.